Field of Application
The present invention relates to a decoder apparatus for use in a television receiver, for processing a bandwidth-compressed high-definition television signal, and in particular to a decoder apparatus for processing a MUSE (multiple sub-Nyquist sampling encoding) system television signal.
The MUSE system has been developed by NHK (Japan Broadcasting Corporation) in order to enable a high definition television signal (abbreviated in the following to HDTV signal) to be bandwidth-compressed to a MUSE signal, which can be transmitted via a channel of approximately 8 MHz bandwidth of a broadcast satellite. Pixels which are within static regions of the picture and pixels which are within moving regions of the picture are processed by respectively separate (low definition and high definition) systems, but are combined prior to transmission, in accordance with the degree of motion associated with each pixel. These separate systems are provided in the MUSE encoder and in the decoder of the TV receiver. Test broadcasts of this system are currently in progress. The MUSE system has been described in various documents in the past, for example in the Nikkei Electronics (Japan), Nov. 2, 1987, pages 189 to 212, in an article entitled "MUSE transmission system for High-Vision broadcasting using earth satellite", so that detailed description of the overall MUSE system will be omitted in the following.
At the transmitting end of such a broadcast system, an encoder processes the luminance and chrominance components of the HDTV signal to obtain the MUSE signal. The luminance (Y) signal compensation of the HDTV signal is first subjected to A/D conversion using a sampling frequency of 48.6 MHz. FIGS. 1A to 1D are diagrams for illustrating the frequency spectrum that is obtained at various stages of conversion of the original luminance signal to that of the MUSE signal. FIG. 1A shows the original frequency spectrum of the luminance component of the HDTV signal, which has a bandwidth extending from 0 to approximately 22 MHz. Next, inter-field offset sampling is executed, with the resultant frequency spectrum being as shown in FIG. 1B. The static image components of the resultant signal are then subjected to sampling frequency conversion, to change the sampling frequency to 32.4 MHz, then inter-frame offset sampling is applied, to obtain the frequency spectrum shown in FIG. 1C.
The image motion components are limited to a bandwidth of 16.2 MHz, then sampling frequency conversion is executed to a sampling frequency of 32.4 MHz, followed by line offset sub-sampling, with the resultant frequency spectrum being as shown in FIG. 1D.
Respective sample values (i.e. pixel values) of the signal obtained by thus processing the static image components and the signal obtained by processing the moving image components are then combined, one pixel at a time, with the combining proportions being determined by an amount of image motion that has been detected for the image region in which each pixel is located. The resultant signal is then subjected to D/A conversion to obtain an analog signal, which is transmitted (multiplexed together with various synchronizing and control signals) as the MUSE signal. Detection of motion of regions within the field that is currently being processed by the MUSE encoder is executed by a motion detector circuit, for each pixel, based for example upon the magnitude of an absolute value of amplitude change between that pixel in the current field and the corresponding pixel of the preceding field. With a MUSE encoder, a static component processing system is used to process pixels which are within a static region of a field (i.e. which are unchanged in absolute amplitude with respect to corresponding pixels of preceding fields), while a motion component processing system processes pixels which are within a moving region of a field. Output signals from the static component processing system and motion component processing system are combined, one pixel value at a time as described above, in accordance with amounts of movement detected by a motion detection circuit.
In addition to such detection of motion of regions within a field, the MUSE encoder also detects overall motion of the picture conveyed by the current field, such as motion resulting from panning or tilting of a television camera. Such overall motion will be referred to in the following simply as panning motion, while motion of individual regions within a picture will be referred to as area motion. When such panning motion begins, motion vector data expressing the amount and direction of panning motion between each field and the preceding field (inter-field), and also between each field and the corresponding field of the preceding frame (inter-frame) are encoded and transmitted together with the image data of the field, as part of the control signals of the MUSE signal. At the MUSE encoder of the television receiver, the motion vector data are used as described hereinafter to ensure that loss of picture definition does not occur in picture regions which are static (but which exhibit apparent motion due to panning).
As shown in FIGS. 1C and 1D, the high frequency components of the original signal of FIG. 1A are "folded over" to fall within a bandwidth of 8.1 MHz, whereby the transmission spectrum bandwidth is compressed to within 8.1 MHz. Such a bandwidth-compressed MUSE signal is received and demodulated by a HDTV receiver which contains a MUSE decoder.
FIG. 2 is a diagram illustrating the sequence of sample values in the MUSE signal. As shown, sample values in lines of an even-numbered field of a frame are identical in phase to those of the odd-numbered field of that frame, and the sample values of each line are displaced by 180.degree. relative to the sample values of the corresponding line of the succeeding frame. In addition, the sample values in successive lines of a field are alternately phase shifted by 180.degree.. It can be understood that the sample positions of each line will coincide with those of the corresponding line of a field that occurred two frame intervals previously.
FIG. 4 is a general circuit block diagram of an example of a prior art MUSE decoder for use in a HDTV receiver. In FIG. 4, only the circuit portions concerned with luminance (Y) signal processing are shown, with the components which deal with chrominance signal processing being omitted for simplicity of description. The received MUSE signal is applied to an input terminal 1, then inputted to an A/D converter circuit 2 in which it is resampled using a sampling frequency of 16.2 MHz, to be converted to a digital signal. The output signal from the A/D converter circuit 2 is supplied to a de-emphasis circuit 3, whose output is supplied to a noise reducer circuit 4, in which noise components of the received MUSE signal are attenuated. The output signal from the noise reducer circuit 4 is supplied to an inter-frame interpolation circuit 5. The inter-frame interpolation circuit 5 is formed of a changeover switch 6 and a 1-frame delay/motion compensation circuit 7, and serves to execute inter-frame interpolation of sample values (i.e. into positions in the signal from which samples have been eliminated by the MUSE encoder). The inter-frame interpolation is executed as follows. The signal of the current field (i.e. the field of the received MUSE signal that is currently being inputted to the inter-frame interpolation circuit 5) is applied via terminal 6a of the changeover switch 6, in alternation with the signal of the corresponding field of the preceding frame (i.e. last field but one) from the output of the 1-frame delay/motion compensation circuit 7, transferred via contact 6b of the changeover switch 6, to the input of the 1-frame delay/motion compensation circuit 7. The switching operations of the changeover switch 6 are in units of pixels, under the control of an inter-frame subsampling clock signal S1 (at a frequency of 16.2 MHz), which expresses sample point and interpolation point phase information. As a result, for each line of the current field, samples from the corresponding line of the corresponding field of the preceding frame (i.e. the last field but one) are interpolated into the appropriate positions. This can be understood from the diagram of FIG. 3 which shows three successive lines of a field of the output signal from the inter-frame interpolation circuit 5. The output digital MUSE signal from the de-emphasis circuit 3 has a sampling frequency of 16.2 MHz, so that an output signal having a sampling frequency of 32.4 MHz is obtained from the inter-frame interpolation circuit 5 as a result of this interpolation.
With a MUSE signal, as described above, the sample positions within each field are identical to those of the field which precedes it by two frame intervals, i.e. there is a high degree of correlation between the current MUSE signal and the MUSE signal of 2 frames previously. This fact is made use of by the noise reducer circuit 4, which executes noise reduction based on correlation between the output signal from the de-emphasis circuit 3 and the output signal from the 1-frame delay/motion compensation circuit 7 which has been passed through the 1-frame delay/motion compensation circuit 7 twice in succession, and therefore has been delayed by two frame intervals.
As described hereinafter, the 1-frame delay/motion compensation circuit 7 within the inter-frame interpolation circuit 5 also serves to execute inter-frame motion vector compensation, and provides a delay of exactly one frame interval only when no motion vector compensation is in progress. The motion vector compensation is controlled by horizontal and vertical motion vector signals which are collectively indicated as M1 in FIG. 4 and are separated from other control signals contained in the MUSE signal, by the control signal separator circuit 19.
The output signal from the inter-frame interpolation circuit 5 is supplied to a static component processing circuit 8 and (via a selector switch 13a) to a motion component processing circuit 9. The static component processing circuit 8 is made up of a LPF 10, a sampling frequency conversion circuit 11 and an inter-field interpolation circuit 12. The interpolated output signal produced from the inter-frame interpolation circuit 5 is transferred through the LPF 10, which has a cut-off frequency of 12 MHz, and the resultant signal is then resampled in the sampling frequency conversion circuit 11, to execute conversion of the sampling frequency from 32.4 MHz to 24.3 MHz. The output signal from the sampling frequency conversion circuit 11 is then supplied to the inter-field interpolation circuit 12, in which inter-field interpolation processing and inter-field motion vector compensation is executed to obtain an output signal having a sample rate of 48.6 MHz. This inter-field interpolation operation is controlled by an inter-field subsampling clock signal S2 (at a frequency of 24.3 MHz) that is produced from an inter-field sampling control circuit 21. The inter-field motion vector compensation is controlled by horizontal and vertical motion vector signals, collectively designated as M3, outputted from the control signal separator circuit 19. Although circuit 8 is designated as the static component processing circuit, in fact the overall static component processing system is made up of the inter-frame interpolation circuit 5 and the static component processing circuit 8, since only a part of the output signal from the inter-field interpolation circuit 5 is supplied to the motion compensation processing circuit 9 as described hereinafter.
The effect of this static component processing system is to produce an output signal from the inter-field interpolation circuit 12 in which each field consists of a superimposed combination of four successive fields of the MUSE signal, i.e. the current field and the three preceding fields. This ensures that a high degree of resolution can be obtained for stationary regions of the finally obtained display picture.
The output signal from the changeover switch 6 of the inter-frame interpolation circuit 5 is also supplied via the switch 13a to the motion component processing circuit 9. The switch 13a is controlled by the aforementioned inter-frame subsampling clock signal S1 produced from the inter-frame sampling control circuit 20, such as to select from the output signal of the changeover switch 6 only the sample values of the current field (i.e. the field whose signal is currently being inputted to the inter-frame interpolation circuit 5, rather than delayed signal produced from the 1-frame delay/motion compensation circuit 7). Thus the input signal to the motion component processing circuit 9 can be considered to be equivalent to the output signal from the de-emphasis circuit 3, but with noise reduction processing having been applied. This process can be understood by referring again to FIG. 3, in which the relationship between the inter-frame subsampling clock signal S1 and three successive lines of the output signal from the inter-frame interpolation circuit 5 is illustrated. As shown, the inter-frame subsampling clock signal S1 changes in phase by 180.degree. on successive lines. Each time that the inter-frame subsampling clock signal S1 is at the H logic level, the switch 13a transfers a sample value of the output signal from the inter-frame interpolation circuit 5 to the motion component processing circuit 9. It can be understood that the timing of the inter-frame subsampling clock signal S1 is determined such that only those sample values which come from the current field (i.e. have not yet passed through the 1-frame delay/motion compensation circuit 7) are selected to be transferred to the motion component processing circuit 9. The motion component processing circuit 9 is made up of a intra-field interpolation circuit 13 and a sampling frequency conversion circuit 14, with the output signal from the switch 13a being inputted to the intra-field interpolation circuit 13. The resultant intra-field interpolated signal produced from the intra-field interpolation circuit 13 has a sampling frequency of 32.4 MHz, and is transferred to the sampling frequency conversion circuit 14 to be converted to a signal having a sampling frequency of 48.6 MHz.
The output signals from the static component processing circuit 8 and motion component processing circuit 9 are supplied to a signal combiner circuit 15, and are combined therein in respective proportions which are controlled by a motion detection signal that is produced from a motion detection circuit 22. More specifically, for each pixel (sample value) of the current field that is being outputted from both the static component processing circuit 8 and the motion component processing circuit 9, if for example a relatively large amount of motion is detected by the motion detection circuit 22 for that pixel (i.e. indicating that the pixel forms part of a moving region within the current field), then the output value for that pixel produced from the static component processing circuit 8 is multiplied by a relatively small factor in the signal combiner circuit 15, while the output value for that pixel produced from the motion component processing circuit 9 is multiplied by a relatively large factor, and the results are summmed and outputted from the signal combiner circuit 15 as the sample value for that pixel.
The motion detection circuit 22 is configured such as to detect only area motion within a field, and not to respond to uniform (panning) motion of the picture conveyed by a field.
The output signal from the signal combiner circuit 15 is supplied to a low-frequency replacement circuit 17, which replaces a fixed proportion of the low frequency components (i.e. in the range of approximately 0 to 3 MHz) of the output signal from the signal combiner circuit 15 with a corresponding proportion of the low frequency components within that same frequency range from the output signal of the de-emphasis circuit 3. The resultant output signal from the low-frequency replacement circuit 17 is transferred to an output terminal 18, as the output luminance signal from the MUSE decoder.
The output signal from the A/D converter 2 is also supplied to a control signal separator circuit 19, which separates the motion vector signals M1 and M3 from motion vector data contained in the control signal portion of the MUSE signal, and outputs signals M1 and M3 from output terminals 19a, 19d respectively. In addition, the control signal separator circuit 19 separates an inter-frame subsampling control signal, which expresses inter-frame subsampling phase information, from the control signals of the MUSE signal, and transfers that signal from an output terminal 19b. The control signal separator circuit 19 further separates an inter-field subsampling control signal, which expresses inter-field subsampling phase information from the control signals of the MUSE signal, and transfers that signal from an output terminal 19c.
The motion vector signals M1 are supplied to the 1-frame delay/motion compensation circuit 7, for applying motion vector compensation. The inter-frame subsampling control signal from output terminal 19b of the control signal separator circuit 19 is supplied to the inter-frame sampling control circuit 20, for controlling generation of the inter-frame subsampling clock signal S1 based on a 16.2 MHz clock signal that is also inputted to the inter-frame sampling control circuit 20. That 16.2 MHz clock signal and also the 24.3 MHz clock signal which is supplied to the circuit 21 are each generated from a clock signal generating circuit (not shown in the drawing) whose operation is phase-locked with the output signal from the A/D converter circuit 2. The inter-field subsampling control signal from output terminal 19c of the control signal separator circuit 19 is supplied to the inter-field sampling control circuit 21, for controlling generation of the inter-field subsampling clock signal S2 based on the aforementioned 24.3 MHz clock signal.
The output signal from the de-emphasis circuit 3 is also supplied to the motion detection circuit 22 which serves to detect, for each pixel of the current field, motion with respect to one or more preceding fields. For accuracy of detection, this will in general be based on a plurality of preceding fields, in order to counteract effects upon the motion detection of the differing sample positions (i.e. offset) between successive fields of the MUSE signal. However even if that is done, it is impossible to achieve as high a degree of accuracy and reliability of motion detection for the motion detection circuit 22 as that of the motion detection circuit of the MUSE encoder apparatus, which operates on sample values that have not yet been "thinned-out" and offset in position.
In the decoder, each field of the output signal from the static component processing circuit 8 is derived by combining the current field and the three preceding fields. When the picture contains panning motion, i.e. uniform overall picture motion, high resolution of the static picture regions (i.e. static other than for the panning motion) is ensured by phase shifting all of the pixels of each field by an amount and in a direction such as to compensate for the panning motion, to ensure that the successively combined fields will have identical values of image phase, and so obtain maximum resolution for such static picture regions during panning motion. The intra-frame motion vector compensation is applied as follows. The amount of intra-frame motion vector compensation that is applied by the 1-frame delay/motion compensation circuit 7, in the horizontal and vertical directions, is determined by the motion vector signals M1. During the current field, the motion vector signals M1 represent the amount of motion vector compensation that is to be applied to the corresponding field of the preceding frame (i.e. that is to be applied to a signal obtained by delaying the signal of the current field by exactly one frame interval), so that the current field is used as a reference field for this compensation.
FIG. 6A shows an example of the internal configuration of the 1-frame delay/motion compensation circuit 7 of FIG. 4. The output signal from the changeover switch 6 (obtained by interpolation of sample values from the corresponding field of the preceding frame into the current field) is applied to an input terminal 23, then through a fixed delay element 24 (e.g. a delay line unit) to a multi-stage delay section 25. This consists of a plurality of delay elements, each providing a delay of 1 H (i.e. one horizontal scanning period). The outputs from the respective stages of the delay section 25 are applied to respective input terminals of a selector switch 26. The switch 26 selects the output from one of these stages of the delay section 25 under the control of a vertical motion vector signal (which is one of the motion vector signals M1) that is applied to an input terminal 27. Thus, an amount of delay (phase shift) in the vertical scanning direction of the television picture is applied, in accordance with the value of the vertical motion vector signal. The output signal from the switch 26 is applied to a shift register circuit 28, with respective outputs from the stages of the shift register circuit 28 being supplied to corresponding input terminals of a selector switch 29. The selector switch 29 selects an output from one of the stages of the shift register circuit 28 (i.e. determines an amount of phase shift applied in the horizontal direction of the television picture) in accordance with the value of a horizontal motion vector signal (the other one of the motion vector signals M1) that is applied to an input terminal 30. The horizontal phase shifting is executed in units of pixel periods, i.e. 10.sup.-6 /16.2 sec.
The resultant motion vector corrected output signal is transferred from an output terminal 31 to the input terminal 6b of the changeover switch 6. When motion vector processing is not being executed (i.e. while the motion vector signals are each at the zero level), the circuit of FIG. 6 is configured such that the total amount of delay between the input terminal 23 and the output terminal 31 is exactly one frame interval. The multi-stage delay circuit 25 in the example of FIG. 5 has a total of 8 stages, and the shift register 28 has 16 stages, e.g. allowing vertical phase shifting in a range of from +4 lines to -3 lines, and horizontal phase shifting in a range of +8 to -7 sample positions.
FIG. 6B illustrates the internal configuration of the inter-field interpolation circuit 12 of FIG. 4. The phase shifting section 82 consists of the blocks 25, 26, 28, 29 shown in FIG. 6A, however the value of the fixed delay provided by delay element 80 is such that a delay of exactly one field interval is produced between the input terminal 81 and the output from the phase shifting circuit 82 when each of the horizontal and vertical motion vector signals is at the zero level. An inter-field interpolation filter 85 receives the input signal of the 1-field delay element 80 and the output signal from the phase shifting circuit 82, and is controlled by the clock signal S2. The output signal from the inter-field interpolation filter 85 is supplied to the combiner circuit 15 of FIG. 4.
The basic operation of the static component processing system in FIG. 4 is as follows. Considering four successive fields of the MUSE signal, designated as n.sub.1, n.sub.2, n.sub.3 and n.sub.4, with n.sub.4 being the current field, the field n.sub.3 is first combined with the delayed field n.sub.1 in the inter-frame interpolation circuit 5, after any necessary inter-frame motion vector phase shifting has been applied to the field n.sub.1 in the 1-frame delay/motion compensation circuit 7. The resultant first combined field is transferred to the inter-field interpolation circuit 12, the same process is executed for fields n.sub.2 and n.sub.4 (with phase shifting for motion vector compensation being applied to field n.sub.2) to obtain a second combined field, then the first and second combined fields are combined in the inter-field interpolation circuit 12, after phase shifting for any necessary inter-field motion vector compensation has been applied to the first combined field. The resultant signal of the combined field from the inter-field interpolation circuit 12 is then transferred to the signal combiner circuit 15, and if any sample value of that signal is found to be part of a moving region, then the value is partially or completely replaced by a sample value which is being produced from the motion compensation processing circuit 9 at that time.
However with the prior art decoder apparatus of FIG. 4, a problem arises when the motion detection circuit 22 exhibits detection errors, or has insufficient accuracy of detection. For example, taking the simplest case in which a single region of the picture is in motion, the pixels of that region will be obtained (i.e. as the output signal from the signal combiner circuit 15) mainly from the output signal of the motion component processing circuit 9, if accurate motion detection is achieved by the motion detection circuit 22. However if the motion detection circuit 22 fails to detect this area motion, then each pixel of the moving region in the output signal from the decoder will be derived (via the signal combiner circuit 15) from the output signal of the static component processing circuit 8. In actual practice, the motion detection circuit 22 may only detect the motion of that region intermittently. When that occurs, the pixels of the region will be correctly derived from the motion component processing circuit 9 output signal during some fields of the output signal from the signal combiner circuit 15, and incorrectly derived from the output signal of the static component processing circuit 8 during other fields. This will result in an unnatural flickering movement of such a moving region on the displayed picture that is obtained, which is conspicuous and objectionable. Such intermittent detection errors by the intra-field interpolation circuit 13 are liable to occur when there is a change in the speed of a moving region within the picture.
As stated hereinabove, it is extremely difficult to achieve accurate operation of the motion detection circuit 22, since the input signal of that circuit consists of sample values which are offset between successive frames. FIGS. 7A, 7B are conceptual timing diagrams for describing the effects of such errors in detecting motion of regions. The vertical lines n.sub.1, n.sub.2, n.sub.3, n.sub.4 represent four successive fields of the input MUSE signal, and the differences between these line positions along the time axis correspond to successive positions of an arbitrary region within the fields. These time axis positions will be referred to as respective image phase values, which are mutually separated by one field interval. n.sub.4 will be assumed to be the current field, n.sub.3 the immediately preceding field, n.sub.2 the last field but one, and so on. The effect of the static image processing system made up of the inter-frame interpolation circuit 5 and static component processing circuit 8 is to generate each field of the output signal from the static component processing circuit 8 as a combination of four successive fields of the input MUSE signal, i.e. each output field represents an image that is a combination of the images expressed by the four fields n.sub.1 to n.sub.4. Thus the image phase of that output field from the static component processing circuit 8 will correspond to a position that is midway between those of n.sub.2 and n.sub.4, along the time axis, as indicated in FIG. 7A. If it is assumed that a single moving region exists in the picture expressed by these fields, i.e. is contained in each of these four fields, then the central position of that moving region, as it appears within the picture expressed by the output combined field produced from the static component processing circuit 8, will be midway between the positions of that region in fields n.sub.2 and n.sub.3, i.e. will have an image phase that is midway between n.sub.2 and n.sub.3. If there is failure to detect that moving region by the motion detection circuit 22, then the region will appear in the resultant displayed picture as a set of pixels that have been outputted from the static component processing circuit 8. On the other hand, if that moving region is detected correctly by the motion detection circuit 22, so that the corresponding portion of the output field from the signal combiner circuit 15 is derived from the output signal of the motion component processing circuit 9, then the corresponding image phase will be that of n.sub.4. This is displaced from the aforementioned central image phase position by 1.5 field intervals, as shown in FIG. 7A, so that there will be a difference between the position of the moving region in the output field from the signal combiner circuit 15 in that case, by comparison with the case in which the pixel data for the moving region are derived from the static component processing circuit 8. The amount of that position difference will of course depend upon the speed of motion of the moving region. However if the motion detection circuit 22 achieves correct motion detection in an intermittent manner, as often occurs in practice, then the moving region will appear to move in an unnatural manner, conspicuously jumping forward or backward for example, in the displayed picture that is obtained. This has been a serious problem in the prior art.
It might be thought that this problem could be easily overcome by simply delaying the input signal applied to the motion component processing circuit 9 by one frame interval, i.e. to input the field n.sub.2 to the motion component processing circuit 9 rather than the current field n.sub.4. If that were done, then as indicated in FIG. 7A, the amount of image phase difference could be reduced to half of a field interval, i.e. the timing difference between the central phase value (indicated by the vertical broken line) and that of field n.sub.2. However although this could be done if only processing of static regions and moving regions were executed, without motion vector compensation, in fact it is not a practical solution, since it is ineffective while panning motion of the displayed picture is in progress.
This will be described referring to FIG. 7B. Specifically, when motion vector compensation is applied, the effect is to bring the field n.sub.1 into image phase coincidence with field n.sub.3, and to bring field n.sub.2 into image phase coincidence with field n.sub.4, then to bring the resultant field n.sub.3 into image phase coincidence with the resultant field n.sub.4, as indicated by the curved arrows in FIG. 7B. In that case, the image phase of a resultant field (a combination of four successive fields) that is outputted from the static image processing system, i.e. from the static component processing circuit 8, will be that of field n.sub.4. Thus, if the aforementioned 1-frame delay were to be applied to the input signal of the moving-image processing system, i.e. to input the field n.sub.2 rather than the current field n.sub.4, then the same problem would arise. In that case, the image phase of the aforementioned moving region as represented in the output signal from the motion component processing circuit 9 (that of field n.sub.2, with no motion vector compensation having been applied) will be significantly different from that of the same moving region as represented in the output signal from the static component processing circuit 8 (having the image phase of field n.sub.4). Thus, simply supplying a 1-frame delayed signal to the motion component processing circuit 9 will not solve the problem.
The above points can be more clearly understood from the pictorial examples of FIGS. 8A and 8B. In FIG. 8A, 63 denotes the current field n.sub.4, and 60 to 62 denote the three preceding fields n.sub.1 to n.sub.3, with the picture expressed by each field containing a fixed region 65 (i.e. part of a stationary background) and a moving region 66, and with the moving region 66 moving horizontally relative to the fixed region 65 in successive fields as illustrated. When these are combined by inter-frame and inter-field interpolation as described above, the picture represented by the resultant combined field will be as indicated by numeral 64. The respective stationary regions of fields n.sub.1 to n.sub.4 are thereby combined to obtain the static region 65'. If the moving region 66 is correctly detected by the motion detection circuit 22, then the pixels representing that region will be obtained from the motion component processing circuit 9, and inserted into the combined field 64, and the resultant region will appear as indicated by 66b. However if motion detection failure occurs, then a combined region determined by the respective positions of the moving region 66 in the fields n.sub.1 to n.sub.4 will be obtained as pixels derived from the static component processing circuit 8, appearing as indicated by 66a in the combined field 64. The center position 67 of that region is substantially displaced from the center position 68 of the region 66b.
FIG. 8B illustrates the case in which horizontal panning in the direction indicated by the arrow has occurred, so that a fixed region 75 moves horizontally in successive ones of the fields n.sub.1 to n.sub.4. At the same time, a moving region 76 is moving relative to the fixed region 75, in a similar manner to the relationship between regions 66 and 65 in FIG. 8A. When these are combined, with motion vector compensation applied as described hereinabove, the position 75' of the fixed region 75 will be that of the region as it appears in the current field n.sub.4, i.e. field n.sub.4 constitutes the image phase reference field as described above. If the moving region 76 is correctly detected by the motion detection circuit 22, then that region will appear at position 76b in the combined field 75'. If failure of motion detection occurs, then the successive positions of the moving region 76 relative to the fixed region 75 in fields n.sub.1 to n.sub.4 will result in the extended region 76a appearing in the combined field 74, representing that moving region. If on the other hand the input signal to the motion component processing circuit 9 had been delayed by one frame interval, i.e. if signal n.sub.2 had been used as the input signal to the motion component processing circuit 9, then the moving region 76 would appear at position 76c in the combined field 74. As can be seen, the center position 76c in that case is substantially different from that of the combined region 77, so that no advantage would be gained by this.
It can thus be understood that the above problem resulting from detection errors of the motion detection circuit 22 does not have a simple solution. That is the first problem of the prior art to be overcome by the present invention.
A second problem which arises with the prior art MUSE decoder is a result of the first problem described above. That is to say, the inter-field interpolation circuit 12 will in practice consist of a combination of a 1-field delay element which receives the output signal from the sampling frequency conversion circuit 11, and a digital filter circuit for combining the input and output signals of the 1-field delay element, to execute inter-field interpolation under the control of the inter-field subsampling clock signal S2. Ideally, that digital filter circuit should be a 2-dimensional type of filter (e.g. formed of a vertically extending array of transversal filters whose output signals are summed), with the filter coefficients being determined such as to provide a 2-dimensional filter response which falls linearly from a maximum value of vertical resolution at low values of horizontal frequency, i.e. less than 4 MHz, to zero at a horizontal frequency of approximately 24.3 MHz. However if such a 2-dimensional digital filter were to be used as the inter-field interpolation circuit 12, the aforementioned adverse effects which result in the finally obtained picture as a result of erroneous operation of the motion detection circuit 22 will be made more conspicuous, due to the high resolution. For that reason, a simple one-dimensional filter is used in the inter-field interpolation circuit 12 in the prior art, which provides a substantially flat 2-dimensional response characteristic of the form indicated by the hatched-line region (1) in FIG. 5. In FIG. 5, the vertical resolution provided by the filter is expressed as a "vertical frequency" in units of cycles/(TV) picture height, where "cycles" signifies transitions between successive scan lines of a picture. Such a 2-dimensional filter characteristic provides substantially constant, low vertical resolution from a horizontal frequency of zero up to the cut-off frequency of 24.3 MHz, as shown. However ideally, such a filter should provide a substantially higher degree of vertical resolution within a range of low frequencies which correspond to the regions of the frequency spectrums shown in FIGS. 1A, 1B in which there is little-or no "folding over" of the spectrum, i.e. in the (horizontal) low-frequency range of approximately 0 to 4 MHz. To achieve that in a practical MUSE decoder circuit, while avoiding the aforementioned problems that prevent the use of a 2-dimensional filter which would provide enhanced vertical resolution in that low-frequency range, a simple transverse filter is used in the inter-field interpolation circuit 12. That filter determines the overall 2-dimensional response in conjunction with the low-frequency replacement circuit 17, which replaces part of the low-frequency components of the output signal from the signal combiner circuit 15 with low-frequency components of the output signal from the de-emphasis circuit 3. Since that output signal from the de-emphasis circuit 3 has not been subjected to filtering in the inter-field interpolation circuit 12, the vertical resolution of the output signal from the low-frequency replacement circuit 17 within the aforementioned low-frequency range is enhanced, as indicated by the hatched-line region (2) in FIG. 5. Moreover, since the direct output signal from the de-emphasis circuit 3 has not been derived based on the operation of the signal combiner circuit 15 controlled by the motion detection signal from the motion detection circuit 22, the effects of detection errors by the motion detection circuit 22 upon the finally obtained output signal from terminal 18 are reduced.
However since the output signal from the de-emphasis circuit 3 has not yet been subjected to noise reduction (in the noise reducer circuit 4) or to sample value interpolation, the frequency component replacement operation of the low-frequency replacement circuit 17 results in increased noise and degraded horizontal resolution of the finally obtained television picture. Thus, increasing the proportion of the low frequency components of the output signal from the static component processing circuit 8 that are replaced by low frequency components of the output signal from the de-emphasis circuit 3 will result in increased vertical resolution in that low frequency range, but in a lowering of the S/N ratio of the output signal thus obtained. On the other hand, decreasing the proportion of the low frequency components of the output signal from the static component processing circuit 8 that are replaced by low frequency components of the output signal from the de-emphasis circuit 3 will result in the opposite effects, i.e. lowered vertical resolution in that low frequency range, but an improved S/N ratio of the output signal produced from the low-frequency replacement circuit 17.
It is therefore a basic disadvantage of such a prior art MUSE decoder apparatus that, as a result of the need to use a simple type of filter circuit to execute inter-field interpolation, it is extremely difficult to obtain a satisfactory degree of vertical resolution together with satisfactory values of noise level and horizontal resolution in the finally obtained television picture. This is a second problem of the prior art to be overcome by the present invention.